Combustion apparatus



Oct. 18, 1955 A SHARPE COMBUSTION APPARATUS Filed July 18. 1951 mm ,wwwmw i,

Inv for 2,

nited States Patent CoMBUs'HoN APPARATUS Alan Sharpe, Ellwood,Melbourne, Australia, assignor to Power Jets (Research and Development)Limited, London, England, a British company Application `luly 1S, 1951,Serial No. 237,375

Claims priority, application Great Britain July 27, 1956 s claims. (el.str-39.71)

This invention relates to combustion apparatus in which combustion ofliquid fuel is required to be supported by a stream of air or other gas(hereinafter referred to as air) owing through a duct with aflame-extinguishing velocity and its primary object, stated in generalterms, is the provision of such an apparatus which will oder thepossibility of eiective and eilicient combustion over a wide range ofoperating conditions. The invention has particular application to thesupporting of continuous combustion by means of such a fast moving aircurrent involving a large mass flow, as for example, in gas turbine orother jet propulsion power units and in gas turbines for other purposes.

The expression flame-extinguishing velocity is used here to indicatethat the mean speed of the stream in its general direction of flowthrough the duct, calculated from the ratio of the air volume passing inunit time to the cross-sectional area of flow path, is substantiallyhigher than the speed of ame propagation in the fuel-air mixtureconcerned. For hydrocarbon fuels burning in air the speed of llamepropagation is considered as being of the order of one foot per secondat atmosphere temperature; the invention, on the other hand, isespecially applicable to combustion apparatus for so-called ram-jetpropulsion power units, as well as to other jet propulsion units and togas turbine power units in general, in which the general velocity of theair stream calculated on the basis indicated, might be of an order aslow as l or as high as 500 feet per second or even more, depending onthe design.

Satisfactory operation of a combustion system of the kind indicated overa wide range of air mass flow and density requires that the llame shouldnot be extinguished under any conditions of operation and in order toprevent this the range of air-fuel ratios over which burning will takeplace must be as wide as possible whilst maintaining combustion eiciencyat a reasonable level with weak mixture. Further it is desirable thatpressure losses should be low and that an even temperature distributionover the cross section of the flow should be achievable. Apart from thenecessity for providing some means of preventing the flame from beingblown out by the air ow, these requirements involve amongst other thingsthe attainment of a high standard of mixing as between the combustionair and the fuel to be burnt, and of atomization and distribution in thecase of a liquid fuel. A common practice in gas turbine and similarcombustion systems is to inject liquid fuel droplets into an almoststagnant region which is protected by a baiile from the full blast eiectof the air ow, specially designed atomizing injection nozzles, or impactsurfaces arranged in the path of the fuel jet, being provided to achievesatisfactory atomization of the fuel; fuel once ignited in this stablezone will continue to burn and ignite further incoming fuel.

According to the present invention, there is provided a combustionapparatus in which combustion has to be supported by a ducted air ow offlame-extinguishing velocity and in which means are provided for formingin the flow a stabilized llame wherein fuel droplets and a 2,720,753Patented Oct. 18, 1955 ICC part of the airstream constituting primaryair are led into the combustion chamber through a mixing duct, the flowof the air-fuel mixture so formed is reversed in direction and ledthrough a vaporizer around said mixing duct, and the flow of the mixtureis then again reversed in direction and led to a primary combustion zonein the combustion chamber to form a flame around said vaporizer; furtherair from the main airstream may mix at high velocity with the burninggases at a region downstream of the primary combustion zone.

In this way the heat of the burning gases serves to vaporize the fuel,most of this vaporization taking place within the vaporizing duct, whileat the same time the axial length of the apparatus is minimized.

In a preferred form of the invention, the mixing duct is co-axial withand occupies a central region of the main duct carrying the mainairstream and is in the form of a diffuser whereby the velocity of theprimary air is reduced as it moves downstream through said mixing duct,fuel being injected (preferably downstream) into said mixing duct at theregion of its upstream end, i. e. at a region of high velocity. At thedownstream end of this duct the ow of unburnt fuel and primary air isreversed in direction to pass upstream through the vaporizing duct andmay be accelerated on its path through this duct by the provision ofswirl vanes therewithin.

At the upstream end of the vaporizing duct the ow of the fuel-airmixture is again reversed in direction and led into a primary combustionzone in the combustion chamber around the vaporizing duct, and at thisregion the velocity of the fuel-air mixture is greatly reduced to assistflame stabilization.

The burning gases then ow downstream from the combustion zone towardsthe outlet to the main duct and are preferably accelerated so that theyfinally emerge into the main duct at high velocity. Downstream of thisoutlet the remainder of the main airstream mixes at high velocity withthe burning gases to complete combustion and/ or to cool the gases.

A detailed view of the combustion apparatus according to the invention,in axial section, is shown in Fig. 1 of the accompanying drawing Whileexternal views of a cylindrical combustion apparatus and an annularcombustion apparatus respectively are shown in Figs. 2 and 3.

In this drawing the main duct may be a cylindrical tube 1 as in Fig. 2or an annular duct 21 as shown in Fig. 3. The device according to theinvention is a pilot burner located in the duct upstream of the mainfuel supply. The pilot fuel supply connection is shown at 22, the mainfuel supply connection at 23 and the terminals of an ignitionsparking-plug at 24 in all iigures. The inlet for the air (or othercombustion-supporting gas) stream is at the left-hand end of each iigureand the outlet for combustion product gases at the right-hand end.

Figs. 2 and 3 are each partly broken away to show the interior; Fig. 1is an enlarged sectional view showing how this revealed part looks inboth cases.

The main airstream carried in the main duct 1 is divided into two partsby a tubular combustion chamber l1 mounted concentrically therein, partof the air flowing through an annulus 2 between duct 1 and thecombustion chamber while another part of the air ows through the pilotburner device incorporated in the combustion chamber 11. The maincombustion zone is dened by the main duct 1 downstream of the main fuelinjector 15, the upstream limit of this zone being indicated at 25.Concentrically mounted within the combustion chamber 11 is a tubularmixing duct 3 which is a diffuser progressively increasing in diameterfrom the inlet at an upstream position in the main duct 1 to the outletfurther downstream. The combustion chamber is open at the downstreamend; the mixing duct 3 is shown as con- 3 s stitutcd by aconcentrirdivergent re-entrant portion of the combustion chamber 11,Yjoined to the outer Wall of chamber 11 by the smoothlj curved end wall9 at the end ojf the inlet tothe mixing duct 3. For convenience theinjector 4 can be of the ordinaryswirl atomizer type, because theconical jet of swirling droplets which issues from this type gives agood distribution of the droplets Y across the cross-section; this gooddistribution is' important although with this invention a high degree ofatomization is not necessary. v y

` When the ir'stl'er'n" Of- VelCity enfers the lefthand end of the mainduct I, fuel isinjected' into thisV stream at a region of high velocityby the injector 4 having the previously-mentioned fuel connection 22. Y*As the mixture of unburnt fuel drops and air flows downstream throughthe' interior of the diffuser duct 3, the

fuel and air become well mixed together and at the downstream end of'duct 3 the ilow of the fuel-air mixture is reversed in direction bymeans of curved'walls 5 which lead the mixture into the annularvaporizer pas-- sage 6 formed betweenY the vaporizing duct 7 and themixing duct 3. Within the Vannulus 6 the vanes` S are swirl vaneswhereby the fuel-air mixture is rotated around the axis of the systemon' its path upstream.

At the upstreamV end of the annular passage 6 the ow of the fueleairmixture is again reversed by means of the curved walls 9 which define aprimary combustion zone -10 in which the' velocity of new is reduced sothat a stabilized llameV can beV formed, 'as explained below. From herethe burning gases can travel downstream again through the space I2 inthe cor'nbustionV chamber outside the vaporizing duct 7. YThusv itl willbe seen that4 the burner device constituted by partsv 3', 7 andlllf,defines a flow path which, as shown by arrows in the drawing, passesValong the interior of *the*l duct 3, then reverses direction'to passalong the interior 6 of the vaporizing Vduct 7 gvnally the path'reverses again into and along the the space 12 in the combustion chamber1'1, and so rcernerges in theV main duct, where combustion products fromvthe combustion chamber will meet the part of the airstream which owsover the outside* of the biirnei' device through the space yFnrtherrore,it will be seen that the'cross-sectional area of the path, afterincreasing along the mixing duct 3, decreases on entering the space 6and then increasesconsiderably again on entering the space 10 ,inV thecombustion chamber; in the v example shown in the drawing theeross-sectional area ofthe space 6 at all sections isless than that atthe outlet from duct 3 while the cross-sectional area of the ow path atthe beginning of space 10 is greater than that at the outlet fromVduct3. Thus in one typical case, when the general air velocity in duct-1, andhence the velocity at the inlet to' duct 3; is 550 ft; per second,the

'velocity decreases to v100 ft. `plerrs'econd at' the outlet from'theduct 3, increases from 200 ft'. per second to 260 ft. per second duringpassage' through the space 6, and decreases still more to V28 ft. persecond on entry into the primary combustion zone 10. Y

.Now although a velocity of 28 ft. per second in a steady stream wouldbe too to maintain stable cornbustion it is found that in the' space'IO,with a reduction to this speed and a complete reversal' ofV direction,conditionsV are formed which allow stable combustion to be maintainedthere or more particularly. at the entrance to the space 12. Once themixture has been ignited here in any way, combustion will belmaintained: in the spaceV 12; the flame will heat the vaporizing duct 7,to ensure substantially complete vaporization of the liquid fuel whichwill continue to be ignitedfas it emerges from the space 6 into thestable zone of combustion;l For convenience in initially starting thecombustion an ignition sparkingplug 14 may be provided where shown, andhaving terminals 24 as already mentioned. The Variation in cross-sectionof the space 12 isA such as to increase the velocity of the burninggases as Vthey move downstream towards the outlet 13 of the combustionchamber' 11 where the burning gases mix with the remainder of theairstream owing atY high speed through the spaceZ and combustion isYcompleted in the downstream portion of the main duct 1. Y n Y Forcompactness, the vaporizing duct 7 is of Vsubstantially the same lengthas the mixing duct 3 so that the outlet from the vaporizing duct intospace 10 is in theY same region as and disposed around the inlet vto themixing duct 3; the combustion chamber 11 is also of about the samelength, the outlet end of the mixing duct 3 and the closed end of thevaporizing duct 7 being both adjacent to the outlet end 13 of thecombustion chamber. y

The flame issuing from the'outlet 13 serves as a pilot flame for mainfuel injected into the main duct 1, for example from a series of fuelnozzles provided symmetrically around the outlet 13 by the annular fueltube 15 having the previously-mentioned fuel-supply connection 23 andshown with three sets of nozzle'apertures 15a, i5bv and 15c Y directingfuel jets perpendicularly across the duct,V upstream and downstreamrespectively; only such of these sets as are desired need be provided.

The combustion apparatus set forth may be used for various purposes. fitmay be the essential part of a ram-jet evice, it may beV connectedbetween the compressor Vand the turbine of a gasturbine plant or in ajetpropulsion gas-turbine plant it may be connected to the end of theexhaust p'ipe of a gas-turbine as a jet pipe, with re-heating by afterburning'.

What I claim is: Y Y l. A combustion device for burning liquid fueldroplets carried in a gas stream, comprising a combustion chamber, .Y avaporising duct within and extending along said combus-V tion chamberand having an outlet into said chamber, and

a mixing duct within and extending along said vaporising duct having atone end an inlet openV to receive the liquid carrying gas stream and atthe other end an outletintoV said vaporising duet, said' chamber andducts together defining a now path for the gas stream which passes alongthe interior of said mixing duct to the outlet thereof, reversesdirection to pass along said vaporising duct outside the mixing duct tosaid outletof the vaporising duct, and then reverses again into andalong said combustion chamber outside said vaporising duct, and ignitionmeans in said combustion chamber for initiating combustion therein andaccordingly, by heat transfer to the gas stream nowing through saidvaporising duct, vaporising thefuel carried therein, wherein saidcombustion chamber and ducts are so dimensioned that the part of saidflow path defined withinV said vaporising duct outsidesaid mixing ducthas in any transverse section plane a ow area less-than either thetransverse ow area of said mixing ducty at the outlet' thereof or theflow area of said combustion chamber in a transverse plane at saidoutlet of the vaporising duct;

2. A combustion device for burning liquid fuel droplets carriedin a gasstream, comprising a combustion chamber, a vaporising duct'rwithin andextending along said combustion chamber and having an outlet intosaidchamber, and a mixing duct within and extending along said vaporisingduct having at one end an inletV open to receive the liquid carrying.gas stream and at the other end an outlet into said Vaporising duct,said chamber and duct together delining a now path for the gas streamwhich passes along the interior of said mixing duct to the outletthereof, re- Verses directed onto pass along said vaporising` duct out-4side the mixing duct to said outlet of the vaporising duct, and thenreverses again into and along said combustion chamber outside saidvaporising duct, and ignition means in said combustion chamber forinitiating combustion therein and accordingly, by heat transfer to thegas stream flowing through said vaporising duct, vaporising the fuelcarried therein, wherein said combustion chamber and ducts are sodimensioned that the part of said ow path defined within said vaporisingduct outside said mixing duct has in any transverse section plane a flowarea less than either the transverse ow area of said mixing duct at theoutlet thereof or the ow area of said combustion chamber in a transverseplane at said outlet of the vaporising duct, and said part of said owpath has swirl vanes provided therein.

3. A combustion apparatus wherein liquid fuel is to be burnt in ahigh-velocity stream of combustion-supporting gas comprising, incombination, of a main duct for the gas stream, a burner deviceconnected to receive part of said stream from one region of the mainduct and to discharge said part into the rest of the ow in a region ofsaid main duct lower down stream, and an injector located in said mainduct for injecting fuel droplets into said part of the stream, whereinthe burner device is made up of a combustion chamber with an outlet enddischarging into the main duct at said last mentioned region, avaporizing duct within and extending along the combustion chamber, and amixing duct Within and extending along the vaporizing duct with an inletend open to receive gas from the main duct at said rst mentioned region,the whole burner device defining a ow path which passes along theinterior of the mixing duct, reverses direction to pass along thevaporizing duct, outside the mixing duct, and then reverses again intoand along the combustion chamber outside the vaporising duct and thenceback into the main duct, said burner device including an igniter in saidcombustion chamber for initiating combustion therein and accordingly byheat transfer to the gas stream owing through said vaporizing duet,vaporizing the fuel carried therein.

4. A combustion apparatus according to claim 3 in which said burnerdevice is a pilot burner and there is a main injector located to injectfuel droplets into the main duct in the path of the gas rejoined by gasfrom the said pilot burner.

5. A combustion apparatus according to claim 3 having the said burnerdevice located within and extending along the main duct, the inlet endof the mixing duct directed upstream and the outlet end of thecombustion chamber directed downstream, with clearance space around theoutside of the burner for the passage of that part of the gas streamwhich does not go through the burner.

6. A combustion apparatus in which combustion of liquid fuel is to besupported by a high-velocity gas stream, comprising, in combination, ofa main duct for the gas stream, a cylindrical burner device lyingcentrally along the said main duct to afford one path for part of thegas through the burner device and another path for the rest of the gasover the outside of the burner device, and a fueldroplet injector withinthe main duct in the region of the upstream end of the burner device,the burner device being made up of a tubular combustion chamber open atthe downstream end, a tubular mixing duct, open at the downstream andupstream ends, constituted by a concentric, divergent, re-entranttubular portion of the combustion chamber, and a vaporizing tube, closedat the end thereof downstream with respect to the direction of ow of thegas stream in said main duct but open at the upstream end,concentrically within the combustion chamber enveloping the mixing ductbut with said closed downstream end spaced from the mixing duct, theinterior of the burner device thus defining a flow path from the mainduct downstream through the mixing duct with progressively increasingcross-section, then upstream, with decreased crosssectional area,through the vaporizing duct, reversing again, with increase ofcross-section to an area greater than the cross-sectional area at theoutlet from the mixing duct, into the combustion chamber and therealonguntil re-emerging into the main duct, the burner device including anigniter in said combustion chamber for initiating combustion therein andaccordingly, by heat transfer to the gas stream flowing through thevaporizing duct, vaporizing the fuel carried therein.

References Cited in the le of this patent UNITED STATES PATENTS GreatBritain .Tune 25, 1948

